Most people understand that the aperture allows light into your camera, which makes its way to the image sensor, where the light is converted and recorded into a picture through a circular window that widens and contracts. Pretty simple right?
The numbers are where people tend to get confused. We measure our aperture numbers in f-stops, and one would typically assume that the bigger the f-stop, the bigger the window, but itís actually the opposite way around. If you have a large number such as f22, this is going to give you a very small window (not much bigger than a pinprick), which wonít let much light in. If you have a small number such as f1.2, the window will be wide open.
The aperture is also responsible for our depth of field (DOF), whereby the portion of the image in focus is altered by the size of the aperture number. The smaller the aperture number, the shallower the depth of field. The larger the number, the deeper the depth of field.
All lenses are going to come with a different minimum and maximum aperture number. The minimum number is usually the one that interests people and is used in the name of the lens. If a lens has more than one number, like 1:3.5-5.6 for example, this means that at its shortest zoom length, this lens can open as wide as f3.5, but at its longest zoom length, the lens will only open to as much as f5.6. Typically lenses with only one minimum number are more expensive, but better quality. You will also find that the smaller the aperture number, the more expensive it is likely to be.
Putting it all together
If youíre just starting out with manual mode, you may not be sure what aperture numbers to use and choose for different situations. So hereís a general guide to get you started:
For shooting street scenes, architecture, landscapes or anything with quite a few subjects that need to be in focus, an aperture number of between f6.3-f18 is ideal. This way, your aperture Ďwindowí isnít so large that you have too much light and too little in focus.
For shooting singular portraits of people, an aperture of between f4-f8 should be enough to ensure your subject is well in focus, but you arenít compromising light. While itís not uncommon for people to shoot portraits below f4, for the dramatic bokeh effect that it creates in the background, just be careful that you arenít missing focus on any important facial features. A headshot taken at f1.2 for example, may only focus on the personís nose and leave their eyes blurry. We always want to make sure that the eyes are sharp.
For shooting a singular flower in a bunch of flowers or anything where you want to narrow your focus right down, an aperture number of f4 or below will give you the best shallow depth of field.